1. Field of the Invention
The present invention is related to a method and an apparatus for injection molding of thermoplastic parts which require very high quality, such as optical lenses and optical disks.
2. Description of the Related Art
Optical lenses and optical disks are frequently injection molded from a thermoplastic material. Due to the optical nature of these parts, however, as it is imperative that the molded parts have a very high mold quality as well as very fine surfaces, conventional injection molding techniques are not acceptable. Any imperfections in the molded part can destroy its usability. Moreover, injection molding of parts such as optical lenses which can contain extreme variation in thickness from an edge of the part to a center of the part, and specifically parts which are very thin toward the center and thick around the outside edges thereof, can present significant difficulties in the molding process. If the edge periphery of the part is many times greater than the thickness in the central area of the part, the part can be very difficult to mold due to the fact that molten material will follow the path of least resistance and travel around the thicker outer edge of the part, creating a void or imperfections in the center portion thereof. An edge thickness to center thickness ratio of 5:1 or 6:1 can be impossible to mold with conventional injection molding techniques.
Typically, molded parts such as lenses having such a thickness ratio are "gated" from one side. This allows injected material such as molten plastic material to flow across the mold cavity area in such a way as to push air or gas in the cavity in front of the molten material toward the opposite side of the cavity. The initial molten material entering the mold cavity, plus the entrapped air or gas, are pushed beyond the opposite edge of the molded part area of the cavity into an overflow well or tab. This configuration is known to dramatically reduce or eliminate weld lines within the parts, and can eliminate or reduce entrapped air or gas within the material which forms the actual part. The gate material and the overflow tab material remain attached to the part after being ejected from the mold, and can be subsequently trimmed off.
In the case of rounded parts such as optical lenses, however, where the edge portion is much thicker than the center portion, the injected material following the wider edge portion more rapidly completes the circle through the thicker edge portion, and can entrap air or gas within the center of the part. A vent relief pin cannot be used for optical parts, since an unacceptable pin mark would remain in the center portion of the part. It has been known that one way to avoid the creation of this "void" is to utilize a method of injection molding wherein the central thickness area of the cavity is temporarily enlarged in the molding process, so that the cavity is enlarged in order to permit molten material to appropriately flow over the thinner areas during the fill cycle of the injection process, and then to subsequently clamp or close the mold so that the mold cavities are brought to the desired final size. U.S. Pat. No. 4,828,769, which is hereby incorporated by reference, discloses one such method for injection molding articles such as lenses which utilizes a temporary enlargement of the mold cavity in the injection process. The process is referred to as "coining" or clamp-end compression. According to the method disclosed in the '769 patent, an injection molding machine is provided wherein a stationary platen supports a stationary or "A" half of an injection mold, and a movable platen which supports a movable or "B" half of the mold. FIG. 1 illustrates a mold system of the present invention disposed in a conventional injection molding machine 1. As shown in FIG. 1, movable platen 10 is moved by a moving device such as hydraulic piston 11, and is guided by a series of tie rods 12 toward stationary platen 13. Conventional coining procedures such as that disclosed in the '769 patent require injection of an amount of molten material which is slightly greater than the molten material which is actually required to form the part, in order to fill or substantially fill the enlarged cavity area. A "toggle type" injection molding clamping action then takes place in the '769 patent, wherein further movement of the movable platen occurs after injection, to pressurize the material in the cavities, and to bring the cavities to the desired shape of the finished parts. The excess material in the cavities is driven into pressure relief ports in the mold. The stationary or A-half of the mold is appropriately configured with a sprue bushing, such that the stationary mold half, the sprue bushing, the stationary platen, and the injection nozzle are held in a stationary relationship throughout the entire molding operation. The '769 patent, and the other prior art in the area of "coining", requires a specialized configuration for the movable platen so as to first move the movable platen into a first or softclose position, and then to secondarily move or toggle the movable mold half attached to a clamping plate within the movable mold half, to complete the molding process. This movement of the clamping plate moves mold insert on the movable platen toward the surface on the stationary platen, thereby pressurizing the injection shot in the mold cavity while bringing the mold inserts to that final position. The injection while the movable platen is in the soft-close position enables the molten material to flow over the center, thinner portion of the mold while the mold is expanded. Since the expanded mold has an edge thickness to center thickness ratio which is much lower than 5:1, molten material can easily flow over the center of the mold cavity. The prior art, therefore, requires a complex and specialized mold machine configuration to accommodate the complex movable mold half toward the stationary mold half in the compression stroke.